Higher-dimensional string cosmological model with bulk viscous fluid in Lyra manifold

I. INTRODUCTION Nowadays relativists are interested in theories with more than four dimensional space-times. Alvarez et al. [1], Randjbar-Daemi et al. [2], Marciano [3] suggested that the experimental detection of time variation of fundamental constants could provide strong evidence for the existence of extra dimensions. The extra dimensions in the space-time contracted to a very small size of Planck length or remain invariant. Further, during contraction process, extra dimensions produce large amount of entropy which provides an alternative resolution to the flatness and horizon problem [4]. Misner [5] pointed out that viscosity plays an important role in the formation of galaxy. Viscosity also accounts for the large entropy per baryon observed in the present universe [6]. Banerjee et al. [7] constructed Bianchi type I cosmological models with viscous fluid in higher dimensional space-time. Chatterjee and Bhui [8] obtained exact solutions of the field equations in a five dimensional space time with viscous fluid. Singh et al. [9] obtained exact solutions of the field equations for a five dimensional cosmological model with variable G and bulk viscosity in Lyra geometry. The study of string theory is important in the early stages of the evolution of the universe before the particle creation. Cosmic strings have received considerable attention in cosmology as they are believed to give rise to density perturbations leading to the formation of galaxies [10]. Chatterjee [11] constructed massive string cosmological model in higher dimensional homogeneous space time. Krori et al. [12] constructed Bianchi type I string cosmological model in higher dimension and obtained that matter and strings coexist through out the evolution of the universe. Rahaman et al.[13] obtained exact solutions of the field equations for a five dimensional space time in Lyra Manifold when the source of gravitation is massive strings. Lyra [14] modified the Riemannian geometry by introducing a gauge function into the structure less manifold as a result of which the cosmological constant arises naturally from the geometry.

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Communications in Physics, Vol. 17, No. 4 (2007), pp. 213-220 HIGHER-DIMENSIONAL STRING COSMOLOGICAL MODEL WITH BULK VISCOUS FLUID IN LYRA MANIFOLD G. MOHANTY, G. C. SAMANTA, AND K. L. MAHANTA Department of Mathematics Sambalpur University Jyotivihar-768019, Orissa, INDIA Abstract. Locally rotationally symmetric five dimensional string cosmological model with bulk viscous fluid in Lyra manifold is constructed. This model is obtained for time dependent displace- ment field, i.e. β = β(t) and constant bulk viscous coefficient. Some physical and geometrical properties of the model are discussed. I. INTRODUCTION Nowadays relativists are interested in theories with more than four dimensional space-times. Alvarez et al. [1], Randjbar-Daemi et al. [2], Marciano [3] suggested that the experimental detection of time variation of fundamental constants could provide strong evidence for the existence of extra dimensions. The extra dimensions in the space-time contracted to a very small size of Planck length or remain invariant. Further, during contraction process, extra dimensions produce large amount of entropy which provides an alternative resolution to the flatness and horizon problem [4]. Misner [5] pointed out that viscosity plays an important role in the formation of galaxy. Viscosity also accounts for the large entropy per baryon observed in the present universe [6]. Banerjee et al. [7] constructed Bianchi type I cosmological models with viscous fluid in higher dimensional space-time. Chatterjee and Bhui [8] obtained exact solutions of the field equations in a five dimensional space time with viscous fluid. Singh et al. [9] obtained exact solutions of the field equations for a five dimensional cosmological model with variable G and bulk viscosity in Lyra geometry. The study of string theory is important in the early stages of the evolution of the universe before the particle creation. Cosmic strings have received considerable attention in cosmology as they are believed to give rise to density perturbations leading to the formation of galaxies [10]. Chatterjee [11] constructed massive string cosmological model in higher dimensional homogeneous space time. Krori et al. [12] constructed Bianchi type I string cosmological model in higher dimension and obtained that matter and strings coexist through out the evolution of the universe. Rahaman et al.[13] obtained exact solutions of the field equations for a five dimensional space time in Lyra Manifold when the source of gravitation is massive strings. Lyra [14] modified the Riemannian geometry by introducing a gauge function into the structure less manifold as a result of which the cosmological constant arises naturally from the geometry. 214 G. MOHANTY, G. C. SAMANTA and K. L. MAHANTA The analog of Einstein’s field equations based on Lyra’s geometry in normal gauge as obtained by Sen [15] and Sen and Dunn [16] are Rij − 12gijR+ 3 2 φiφj − 34gijφkφ k = −χTij (1) where φi is the displacement vector and other symbols have their usual meanings as in the Riemannian geometry. So far the study of string with bulk viscosity in higher dimensional Lyra manifold is not yet found in the literature. Therefore we have taken an attempt to construct LRS Bianchi I five dimensional string cosmological model with bulk viscous fluid in Lyra manifold. Earlier Bali and Upadhaya [17] constructed four dimensional LRS Bianchi type I string cosmological model with constant bulk viscous coefficient in general relativity. In this paper the energy momentum tensor is assumed to be the simple extension of usual four dimensional cases. II. THE METRIC AND THE FIELD EQUATIONS Here we consider a five dimensional LRS Bianchi type I metric in the form ds2 = −dt2 +A2dx2 + B2(dy2 + dz2) + C2dΨ2 (2) where A, B and C are functions of cosmic time t only. We assume here that the co-ordinates to be commoving i.e. u0 = 1 and u1 = u2 = u3 = u4 = 0 (3) Further we consider here the displacement vector φi in the form φi = (β(t), 0, 0, 0, 0) (4) The energy momentum tensor for a cloud of string dust with a bulk viscous fluid given by Landau and Lifshitz [18], Letelier [19] and Bali and Dave [20] is Tij = ρuiuj − λwiwj − ξul;l(gij + uiuj), (5) where ξ is the bulk coefficient of viscosity, ρ is the proper density for a cloud of strings with particles attached to them , λ is the string tension density, uiis the five velocity of the particles, gij is the covariant fundamental tensor , wi is the unit space like vector representing the direction of the string satisfying uiui = −wiwi = −1 (6) and uiwi = 0 (7) Further the expansion scalar is given by θ = ul;l (8) Using equations (4), (5) and (6) the explicit form of field equations (1) for the line element (2) are obtained as HIGHER-DIMENSIONAL STRING COSMOLOGICAL MODEL WITH BULK VISCOUS FLUID ... 215 − ( B′ B )2 − 2A ′B′ AB − 2B ′C ′ BC − A ′C ′ AC + 3 4 β2 = −χρ (9) 2 B′′ B + C ′′ C + ( B′ B )2 + 2 B′C ′ BC + 3 4 β2 = χ(λ+ ξθ) (10) A′′ A + B′′ B + C ′′ C + A′B′ AB + B′C ′ BC + A′C ′ AC + 3 4 β2 = χξθ (11) A′′ A + 2 B′′ B + ( B′ B )2 + 2 A′B′ AB + 3 4 β2 = χξθ (12) where dash denotes differentiation with respect to t. In the following section we intend to derive the exact solutions of the field equations usingβ = β(t) and ξ = ξ0 (con- stant)(Mohanty and Pattanaik [21], Bali and Upadhaya [17], Bali and Yadhav [22]) in order to overcome the difficulties due to non linear nature of the field equations. III. COSMOLOGICAL SOLUTIONS Here there are six unknowns viz. A, B, C, β, ρ and λ involved in four field equations (9)-(12). In order to avoid the insufficiency of field equations for solving six unknowns through four field equations here we consider the power law i.e. C = Bn (13) where n is a constant. Subtracting equation (11) from (12) we find B′′ B − C ′′ C + ( B′ B )2 + A′B′ AB − A ′C ′ AC − B ′C ′ BC = 0 (14) Substituting equation (13) in equation (14) we get B′ B ( B′′ B′ + (n + 1) B′ B + A′ A ) = 0, (15) which yields following three cases Case I: B ′′ B′ + (n+ 1) B′ B + A′ A = 0 Case II: B′ = 0 Case III: B′ = 0 and B ′′ B′ + (n+ 1) B′ B + A′ A =0. Now in the following subsections we intend to derive the exact solutions of the field equa- tions for the above mentioned cases. III.1. Case I B′′ B′ + (n+ 1) B′ B + A′ A = 0 (16) In this case we find B(t) = [ (n+ 2) (∫ dt A(t) + k )] 1 n+2 216 G. MOHANTY, G. C. SAMANTA and K. L. MAHANTA from which it is clear given any function A(t)we can find a B(t). Therefore the solutions are unique. However for further studies here we consider B′′ B′ + (n+ 1) B′ B = −A ′ A = k(> 0 cons tan t) which yields B = [ (n+ 2) ( k1 k ekt + k2 )] 1 n+2 (17) and A = k3e−kt (18) where k1 6= 0, k2 and k3 6= 0 are constants of integration. Now equation (13) yields C = [ (n+ 2) ( k1 k ekt + k2 )] n n+ 2 . (19) Substituting equations (17) - (19) in equation (11) we get 3 4 β2 = χξ0 [ −k + k1e kt k1 k e kt + k2 ] + (2n+ 1)k21e 2kt (n+ 2)2 ( k1 k e kt + k2 )2 − k2. (20) Putting equations (17)-(20) in equations (9) and (10) we find ρ = kξ0 − k1e kt χ ( k1 k e kt + k2 )(k + χξ0) (21) and λ = kk1e kt χ ( k1 k e kt + k2 ) − k2 χ . (22) The particle density is obtained as ρp = ρ− λ = kξ0 + k 2 χ − k1e kt χ ( k1 k e kt + k2 )(2k+ χξ0). (23) In this case metric (2) takes the form ds2 =− dt2 + k23e−2ktdx2 + [ (n + 2) ( k1 k ekt + k2 )] 2 n+2 ( dy2 + dz2 ) + [ (n + 2) ( k1 k ekt + k2 )] 2n n+2 dΨ2. (24) HIGHER-DIMENSIONAL STRING COSMOLOGICAL MODEL WITH BULK VISCOUS FLUID ... 217 III.2. Case II B′ = 0 (25) In this case we find B = a (constant). (26) Now equation (13) yields C = an (constant). (27) Substituting equations (26) and (27) in the field equations (9) - (12) we obtain 3 4 β2 = −χρ (28) 3 3 β2 = χλ+ χξ A′ A (29) A′′ A + 3 4 β2 = χξ A′ A (30) Here there are four unknowns involved in three equations (28) - (30). In order to get explicit solutions we have to assume a physical or a mathematical condition. Therefore in this case we consider the following cases: ρ+ λ = 0 (Reddy [23, 24]) (31) ρ = λ (geometric strings) (32) and ρ = (1 + ω)λ (Takabayasi string or p− string). (33) III.2.1. ρ+ λ = 0 Subtracting equation (28) from equation (29) and using relation (31) we obtain A = constant (34) Substituting equation (34) in equation (30) we get β2 = 0 (35) Therefore in this case the model (20) reduces to empty flat model of Einstein’s theory. III.2.2. ρ = λ(Letelier [25]) Adding equations (28) and (29) and using relation (32) we find 3 2 β2 = χξ0 A′ A (36) Subtracting equation (36) from two times of equation (30) we obtain A′ A ( A′′ A′ − χξ0 ) = 0 (37) which yields A′ = 0 (38) 218 G. MOHANTY, G. C. SAMANTA and K. L. MAHANTA or A′′ A′ − χξ0 = 0 (39) Using equation (38) we get the same model as obtained in section 3.2.1. However from equation (39) we obtain A = a1 χξ0 eχξ0t + a2 (40) where a1 and a2 are constants of integration. Substituting equation (40) in equation (36) we find 3 2 β2 = χa1e χξ0t a1 χξ0 eχξ0t + a2 (41) Equation (28) with the help of equation (41) yields ρ(= λ) = −a1eχξ0t 2 ( a1 χξ0 eχξ0t + a2 ) (42) Now the solutions given by (26), (27), (40), (41) and (42) satisfy the field equations(9)-(12) only when a1 = 0. This immediately yields ρ(= λ) = 0 β2 = 0 and A = constant. Hence in this case the model is same as obtained earlier in section 3.2.1. Here we mention that in case II the model for p-string could not be obtained due to highly non linear nature of the field equations. III.3. Case III B′ = 0 and B′′ B′ + (n+ 1) B′ B + A′ A = 0 This case is not acceptable due to indeterminacy of B. IV. PHYSICAL AND GEOMETRICAL PROPERTIES In the preceding section the metric (24) represents a string cosmological model with bulk viscous fluid in Lyra manifold. At the initial epoch t = 0, the metric becomes flat. As time increases the rate of expansion in the model along y and z axes are faster in comparison to the contraction of the model along x axes when −2 < n < 0.The extra dimension contracts if −2 < n < 0.The parameters involved in the model behave as follows: (a) The rest energy density for the model (24) given by equation (21) satisfies the reality condition ρ > 0 when kξ0χ ( k1 k ekt + k2 ) > k1e kt(k + χξ0). HIGHER-DIMENSIONAL STRING COSMOLOGICAL MODEL WITH BULK VISCOUS FLUID ... 219 (b) The scalar of expansion for the model is obtained as θ = k1e kt k1 k e kt + k2 − k. At the initial epoch t = 0, θ is finite and θ → 0 when t→∞. The expansion in the model stops at infinite time. Thus there is finite expansion in the model. (c) The shear scalar σ2 for the model (24) is σ2 = 1 2  k21e2kt(n2 + 2) (n+ 2)2 ( k1 k e kt + k2 )2 + k1ekt(n+ 1) (n+ 2) ( k1 k e kt + k2 ) + k2 + k  . Since lim t→∞ σ2 θ2 6= 0 , the universe remains anisotropic throught the evolution. (d) The spatial volume of the universe is V = (n+ 2) ( k1 k e kt + k2 ) k3ekt , k3 > 0 Thus the volume of the universe is finite throught the evolution. (e) The deceleration parameter (Fienstein et al, [26]) q = −3θ−2 ( θ;iu i + 1 3 θ2 ) = − ( 3k1ekt kk2 + 1 ) . For k, k1, k2 > 0, the value of the deceleration parameter is negative, which indicates inflation in the model. V. CONCLUSION In this paper we have constructed a five dimensional string cosmological model with bulk viscous fluid in Lyra manifold. The model obtained is free from initial singularity, which supports the analysis of Murphy [27] that the introduction of bulk viscous fluid avoids the initial singularity. REFERENCES [1] E. Alvarez, and M. B. Gavelo, Phys.Rev.Lett., 51 (1983) 931. [2] Randjbar-Daemi,S.,Salam, A.,Strathdec, J., Phys. Lett. B, 135 (1984) 388. [3] W. J. Marciano, Phys. Rev. Letters, 52 (1984) 489. [4] A. H. Guth, Phys. Rev. D, 23 (1981) 347. [5] C. W. Misner, Astrophys. J., 151 (1969) 431. [6] N. Caderni, and R. Fabri, Nuovo Cimento 44B (1978) 228. [7] A. Banerjee, B. Bhui, and S. Chatterjee, Astrophysical Journal, 358 (1990) 187. [8] S. Chatterjee, and B. Bhui, Astrophys. Space Sci., 167 (1990) 61. [9] G. P. Singh, R. V. Deshpande, and T. Singh, Pramana J. Phys., 63 (2004) 937. [10] Ya. B. Zel’dovich, Mon. Not. R. Astr. Soc., 192 (1980) 663. [11] S. Chatterjee, Gen. Rel. Grav. 25 (1993) 1079. [12] K. D. Krori, T. Chaudhuri, and C. R. Mahanta, Gen. Rel. Grav. 26 (1994) 265. [13] F. Rahaman, S. Chakraborty, S. Das, M. Hossain, and J. Bera, Pramana J. Phys., 60 (2003) 453. 220 G. MOHANTY, G. C. SAMANTA and K. L. MAHANTA [14] G. Lyra, Mathematische Zeitsehrift, 54 (1951) 52. [15] D. K. Sen, Z.Phys., 149 (1957) 311. [16] D. K. Sen, K. A. Dunn, J. Math. Phys., 12 (1971) 578. [17] R. Bali and R. D. Upadhaya, Astrophys. Space Sci., 288 (2003) 287. [18] L. D. Landau, and E. M. Lifshitz, Fluid Mech., 6 (1963) 505. [19] P. S. Letelier, Phys. Rev. D, 20 (1979) 1294. [20] R. Bali, and S. Dave, Astrophys. Space Sci., 282 (2002) 461. [21] G. Mohanty, and R. R.Pattanaik, Int. J. Theo. Phys., 30 (1991) 239. [22] R. Bali and M. K. Yadhav, Pramana J. Phys., 64 (2005) 187. [23] D. R. K. Reddy, Astrophys. Space. Sci., 286 (2003) 365. [24] D. R. K. Reddy, Astrophys. Space. Sci., 286 (2003) 359. [25] P. S. Letelier, Phys. Rev. D, 28 (1983) 2414. [26] A. Feinstein, J. Ibanez, and P. Labarge, J. Math. Phys., 36 (1995) 4962. [27] G. L. Murphy, Phys. Rev. D, 8 (1973) 4231. Received 06 October 2007.